Ultima 3: On the overworld (Sosaria) when you killed monsters they'd drop a chest in the square where they were. The monsters on the overworld could not go into squares with chests in them so I'd make "roads" between all of the sites in the game by having a blank square with a chest on either side. This made it easy to go anywhere in the game without getting annoyed by combat and it was fun to let the monster "kingdoms" formed by the chest fence fill to capacity and then clear them out from time to time.
Me and my friend had a self imposed game mode called "Police Mode", we'd go through the whole game using only the pistol and a shotgun (sniper if it was a "SWAT" mission), we could only fire the shotgun if enemies were considered "heavy". We also had to announce ourselves every time we entered an area, and we could only fire at an enemy if fired upon first.
My buddies and I would spend hours messing around in the final room of the Facility in Goldeneye, mostly just making guys blow up and seeing how far they flew.
this one right here. me and a friend used to spend lots of time just passing the controller back and forth getting stars up and seeing how long we could survive.
i sometimes don't save in skyrim and see how far I can go before the game crash
In both Prototype games I used to take a soldier's form and join the battle against the infected without using my powers. Turned the game into some sort of TPS in survival mode.
I made quite a few games in Epic Games' ZZT back in the day... an ANSI-based game creation system developed by Tim Sweeney back in 1991. Had a ton of fun making them (even though they are super-rudimentary) and it taught me basic programming.